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HE OCCURRENCE OF WHEAT DOWNY 
MILDEW IN THE UNITED STATES 



WILLIAM H. WESTON, JR. 

Pathologist in Charge of Downy Mildew Investigations 




UNITED STATES DEPARTMENT OF AGRICULTURE 
DEPARTMENT CIRCULAR 186 



Contribution from the Bureau of Plant Industry 

(Office of Cereal Investigations) 

WM. A. TAYLOR, Chief 



Washington, D. C. Issued June, 1921 



WASHINGTON : GOVERNMENT PRINTING OFFICE : 1921 



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THE OCCURRENCE OF WHEAT DOWNY MILDEW 
IN THE UNITED STATES. 



A disease of wheat, which, although long recognized in Europe, 
has hitherto been unknown in the United States, has recently been 
found in Tennessee and Kentucky. The disease is not recognized 
as such by the farmers of these States, nor is it given any common 
name by them, but since it is caused by a fungous parasite, Sclero- 
spora macrospora Sacc, one of the downy mildews (Peronospora- 
cese), it may be referred to conveniently as Sclerospora or downy 
mildew of wheat. It is not especially destructive in so far as is 
known at present, but in view of the losses it has caused in Europe 
it should be carefully watched. During the present season it has 
been encountered only in Jackson County (Jackson) and in Obion 
County (Rives, Hoosier Valley, Union City, and Woodward Mills) 
in Tennessee, and in Fulton County (Jordan and vicinity) in Ken- 
tucky, but it seems probable that it will be found in adjacent Illi- 
nois, and possibly across the Mississippi River in Arkansas and Mis- 
souri as well. In addition, diseased specimens in the pathologic 
herbarium at the California Agricultural Experiment Station show 
that the downy mildew occurred on wheat in Kings County, Calif., 
in May, 1919. 

A preliminary study in Tennessee and Kentucky has shown that 
the disease is restricted almost entirely to low-lying, poorly drained 
fields or parts of fields in which the young seedlings of winter wheat 
have been subjected to excessive moisture. In such localities the 
Sclerospore occurs especially along depressions where water has 
stood, while few, if any cases are found on higher land in the same 
field. 

The general indications from field conditions in the infected re- 
gions are that the disease is not of sudden or recent appearance, but 
has been present for many years. Evidences of this are the facts that 
the Sclerospora is widely distributed there, that it occurs commonly 
on wheat from seed grown for many years in this region, and that it 
does not cause a serious outbreak but persists in the low bottom lands, 
causing slight but constant losses. 

The disease is found not only on the three or four varieties of soft 
Red Winter wheat which are grown in these localities, but also on 

54016—21 (3) 



Bromus cummutatus Schrad., a grass similar to " cheat ,? and very- 
common both in the wheat and around field borders. 

With respect to their appearance, the three following general types 
of diseased wheat plants may be recognized in fields not yet mature : 

(1) The diseased plants show excessive tillering, as many successive shoots 
grow up until a few inches high, then rapidly wither and turn brown, resulting 
in a clump of small, closely crowded, dead plants. 

(2) The internodes fail to develop normally; the plants remain short and 
stunted, while their leaves, instead of being dark green and hanging flexibly in 
normal, graceful curves, become striped or almost completely yellow and fleshy. 
The leaf tissue thickens abnormally, causing the leaves to twist or curl and 
stand out in stiff, unnatural positions. 

(3) The leaves are somewhat yellowed, thickened, and twisted, but the growth 
of the attacked plant is not so conspicuously arrested as in the two preceding 
types and continues at a rate only little less than normal. 

These types are, of course, only approximate, and some plants at- 
tacked by the disease do not fit in general appearance into any one of 
them exactly, while others show some characteristics of all three. In- 
fected plants of the "cheat "-like grass {Bromus commutatus) in 
almost all cases are of the second or third type mentioned above. 

The gross appearance of the diseased plants, which has just been 
described, closely resembles that resulting from a number of other 
causes, but there is one unfailing mark by which this Sclerospora 
disease can always be recognized. When leaves of plants infected 
with this fungus are held up toward the sky, so that the light shines 
through them, and are studied carefully with a hand lens or pocket 
magnifying glass, it is seen that within the tissue of the leaf, which, 
when healthy, is a clear solid green, there are innumerable clear dots 
running more or less in rows along the small veins or even in the 
veins themselves. The presence of these innumerable minute dots 
in the leaf tissue is a sure sign that the plant is infected with the 
Sclerospora. When a bit of such tissue is scraped out into a drop 
of water and examined under the microscope, it is seen that these 
dots are the large, clear resting spores or reproductive bodies of 
the fungus. It is these spores that arc set free in the soil by the 
eventual decay of the plant and remain there to bring about the 
infection of future wheat or grass plants. 

The subsequent development of wheat plants attacked by the 
disease varies in the different types. Plants which show the exces- 
sive tillering which characterizes the first type invariably die early 
without reaching maturity. Plants of the second and third types 
struggle along to greater age and occasionally, though rarely, head 
out, producing distorted and abnormal heads with few, if any. 
viable seeds. Infected plants of Bromus, however, usually are less 
severely injured and frequently produce their inflorescences, which 
are only slightly less vigorous and well developed than normal. 



The losses occasioned by this disease on wheat in the west Tennessee 
and Kentucky district apparently are slight. In wet, poorly drained 
localities which favor the disease, large numbers of plants are affected 
and destroj-ed, but as such poorly drained areas represent a very 
small proportion of the whole district and as the disease is closely 
restricted to such localities, the total loss is not very great. This loss 
is persistent, however, for the fungus survives, between wheat crops, 
on Bromus or in the soil and continues year after year, crop after 
crop, taking a toll which, although slight, represents a considerable 
loss in the aggregate. 

Unfortunately, no really practical suggestions for control can be 
made as yet. In Italy, attempts have been made to diminish the dis- 
ease by collecting and burning the infected plants, but with little suc- 
cess. As effective methods of control can be devised only from a thor- 
ough understanding of the very obscure life history of the causal 
Sclerospora, this phase of the problem is now being studied by the 
Office of Cereal Investigations. 

At present it is important that the disease should be watched, be- 
cause of its possibilities for future destruction. Its history in Eu- 
rope, and especially in Italy, where it has been known for more than 
40 years, indicates that it may prove much more troublesome in the 
future. In Italy it has been found especially on wheat and maize, but 
occasionally also on oats, barley, rice, and on several wild grasses of 
the genera Phalaris, Phragmites, Glyceria, Agropyron, and Lolium. 
It is very probable, therefore, that under favorable circumstances the 
disease may attack any of these hosts in our own country. In Europe, 
moreover, there have been occasional reports of very severe destruc- 
tion of wheat and maize which the disease has caused in regions sub- 
ject to flood or excessive moisture. In all cases this loss has been 
sporadic, severe destruction occurring in restricted areas where cir- 
cumstances were favorable, while areas more fortunately situated in 
the vicinity suffered only slight losses. There is every reason to be- 
lieve, therefore, that under conditions favorable to the disease, more 
or less restricted areas of our own wheat or corn raising regions will 
lose a considerable percentage of their crop because of this downy 
mildew. 

In order to determine the extent of the disease, its distribution 
throughout our wheat-growing States, and its occurrence on corn 
and other hosts, there is need of the help of those who are in the 
field or in touch with the field situation during the present season. 
It is hoped that this brief note may direct the attention of path- 
ologists and other investigators to the disease, so that it may be dis- 
covered and reported as soon as possible. In the case of wheat, it 
may be recognized by the characteristic symptoms which have been 



6 

described in this circular and which are well illustrated in a bulletin 
by Peglion (1). In corn, its presence may be noted by the twisted 
and abnormal growth, particularly of the tassel, which is described 
and figured by Ippolito and Traverso (2) and by Gabotto (3). In 
rice also, the plant, and particularly the head, is deformed, as is 
shown in the illustrations of Gabotto (4) . Similar deformations of 
any of these plants, however, might result from other causes; but 
the presence of its innumerable spores in the tissue of the infected 
plant and the pellucid dotted appearance of such tissue when ex- 
amined with a hand lens by transmitted light are sure signs of the 
presence of tlie fungus. The Office of Cereal Investigations of the 
Bureau of Plant Industry is making a study of this disease and will 
be very glad to examine and report on any material which may be 
sent in. 

LITERATURE CITED. 

(1) Peglion, Vittorio. 

1901. La peronospora del fruniento. Nuove richerche. hi Staz. Sper. 
Agr. Ital. v. 34, p. 506-532, pi. 1-3. 

(2) Ippolito Giulio d\ and Traverso, G. B. 

1903. La Sclerospora macrospora Sacc. In Staz. Sper. Agr. Ital. v. 36, 
p. 974-996, pi. 9. 

(3) Gabotto, Luigi. 

1918. La peronospora del mais. In Coltivatore, v. 64, p. 331-333, fig. 23-25. 

(4) 1915. La peronospora (Sclerospora macrospora Sacc.) sul riso. In II 

Gior. di Risicolt, v. 5, p. 292-294, 1 fig. 



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